The sulfated oligosaccharide agent known as PI-88 [1,2] (see compound 1 below) has been shown to be a promising inhibitor of tumour growth and metastasis [1,3] and is undergoing Phase II clinical trials in cancer patients [4]. PI-88 exerts antiangiogenic effects by inhibiting the interactions of angiogenic growth factors (principally FGF-1, FGF-2 and VEGF) and their receptors with heparan sulfate [1,5]. In addition, PI-88 is a potent inhibitor of the enzyme heparanase, a glycosidase that cleaves the heparan sulfate side chains of proteoglycans that are a major constituent of the extracellular matrix (ECM) and basement membranes surrounding tumour cells [1,2]. Heparanase has been strongly implicated in angiogenesis: it is able to liberate active heparan sulfate-bound angiogenic growth factors from the ECM and is involved in the degradation of the ECM and subsequent tissue remodeling associated with the sprouting of new blood vessels [6]. The degradation of the ECM by heparanase is also crucial in the spread of tumour cells (metastasis) by allowing them to pass into the blood stream and lodge in remote sites where they can form secondary tumours [6,7].
In addition to its antiangiogenic effects, PI-88 inhibits the blood coagulation cascade by (i) inhibiting proteases in the intrinsic pathway, (ii) stimulating the release of tissue factor pathway inhibitor (TFPI), and (iii) activating the heparin cofactor II-mediated inhibition of thrombin. However, PI-88 does not interact with AT III and thus shows no anti-Xa or AT III-mediated anti-IIa activity [8,9]. In vivo studies in monkeys have shown that low doses of PI-88 stimulate release of all heparan sulfate bound TFPI from the vascular cell wall [9]. Apart from its effect on coagulation, TFPI was recently shown to be an antiangiogenic agent [10] and an inhibitor of metastasis [11]. PI-88 has also been shown to block vascular smooth muscle cell proliferation and intimal thickening [12], to inhibit herpes simplex virus (HSV) infection of cells and the cell-to-cell spread of HSV-1 and HSV-2 [13], and to inhibit proteinuria in passive Heymann nephritis [14].
PI-88 is a mixture of highly sulfated, monophosphorylated mannose oligosaccharides ranging in size from di- to hexasaccharide [15,16]. PI-88 is prepared by exhaustive sulfonation [2,16] of the oligosaccharide phosphate fraction (2) (see formula I following this paragraph) obtained by mild, acid-catalyzed hydrolysis of the extracellular phosphomannan of the yeast Pichia (Hansenula) holstii NRRL Y-2448 [17,18]. The major components are the penta- and tetrasaccharide phosphates 3 (˜60%) and 4 (˜30%), respectively, whilst the remaining 10% is made up of di-, tri- and hexasaccharide phosphates (5-7) and a tetrasaccharylamine (not shown) [15,16].
Formula I nRR110-4SO3Na or HPO3Na220-4HPO3Na233HPO3Na242HPO3Na250HPO3Na261HPO3Na274HPO3Na280HH91HH102HH113HH
Various other polysulfated oligo- and polysaccharides and their derivatives are well known to exhibit similar types of biological activities to PI-88 [19-25]. These biological activities are attributed to the inhibition of various heparan sulfate (HS)-binding proteins. The object of the present invention is to create derivatives of PI-88 that have similar biological activities but with improved properties, for example, in their pharmacokinetic and/or ADME (absorption, distribution, metabolism, excretion) profiles. A further object of the invention is to provide compounds comprising a single carbon skeleton to facilitate their synthesis and characterization.